Not Applicable.
The present invention relates to a boron carbide semiconductor polytype and a method for fabricating the same. More specifically, the present invention relates to a non-doped n-type boron carbide semiconductor polytype that may be used in the context of detecting the presence of neutrons and electric power conversion.
Neutron scattering is an important research method to determine the structure of solids and liquids. It is used to understand the forces that act between the atoms in these systems and to determine the magnetic behavior of materials as well. The research and practical applications cover a broad range of areas, from the basic properties of materials to studies of engineering and medical applications.
Boron is used in forming many solid state neutron detectors because of the large neutron capture cross-section of 10B. Boron carbide has the ability to withstand high temperatures, corrosive and mechanically abrasive environments, and may also be used in radioactive environments.
In the past, boron carbide/Si (111) heterojunction diodes have been fabricated from closo-1,2-dicarbadodecaborane (C2B10H12-orthocarborane) by using synchrotron radiation induced chemical vapor deposition (SR-CVD) and plasma enhanced chemical vapor deposition (PECVD). While it has been realized that boron carbide exhibits a number of different semiconducting polytypes by using orthocarborane in fabricating the diode, all of them have been found to be nearly perfectly compensated semiconductors or slightly p-type. Since p-type p+ heterojunction diodes generally have less boron containing semiconductor for the purposes of neutron detection or direct powerconversion than a p-n boron-carbide homojunction diode, n-type boron carbide semiconductor materials with n-type properties have been sought for diode fabrication.
Previously, in order to form a n-type boron carbide semiconductor, a metal dopant, such as nickel, was typically introduced into the fabrication process using most boranes, including orthocarborane. In particular, a source gas orthocarborane was used as a source gas to grow the boron carbide, while nickelocene (Ni(C5H5)2) was used to introduce nickel into the growing film. The doping of nickel into the film transforms a p-type B5C material, relative to a lightly doped n-type silicon or p-type boron carbide, into an n-type material.
However, the use of a metal dopant to form a n-type boron carbide semiconductor has numerous drawbacks and deficiencies. For instance, the transition metal dopant can become an undesirable radioactive isotope when exposed to a highly radioactive environment. Furthermore, the use of a transition metal for doping lowers the resistivity of the resulting boron carbide layer from about 1010 ohm centimeters to about 105 ohm centimeters or less thereby reducing the ability of this layer to act as a dielectric material, and may not have long term stabilities at elevated temperatures of about 250xc2x0 C. or higher.
Accordingly, there remains a need for a boron carbide semiconductor polytype and method of fabricating the same that is n-type that will not produce undesirable radioactive isotopes when exposed to a radioactive environment. Further, there remains a need for a n-type boron carbide semiconductor that has adequate dielectric properties. The present invention fills these needs as well as various other needs.
In order to overcome the above-stated problems and limitations, and to achieve the noted objects, there is provided naturally occurring non-doped n-type boron carbide semiconductor polytype and a method of fabricating the same.
The semiconductor of the present invention may be used as heterojunction and homojunction neutron detection devices, as well as in the direct conversion of the neutron flux to current (i.e., direct conversion to electric power). In general, the neutron detection device may include a boron carbide layer coupled with a substrate layer. The boron carbide is naturally n-type and may be an electrically active part of the device.
The natural n-type boron carbide semiconductor of the present invention may be fabricated by decomposing closo-1,7-dicarbadodecaborane (C2B10H12-metacarborane). Specifically, the natural n-type boron carbide semiconductor polytype of the present invention may be fabricated using synchrotron radiation induced chemical vapor deposition or electron beam induced chemical vapor deposition. This process includes the steps of providing for a substrate and metacarborane, placing the substrate in a vacuum chamber, cooling the substrate, introducing metacarborane into the vacuum chamber, adsorbing the metacarborane onto the surface of the substrate, and through the use of incident X-ray radiation or electron beam flux, decomposing the adsorbed metacarborane, and allowing the substrate to reach ambient temperature.
Furthermore, the boron carbide polytype of the present invention may also be fabricated by using plasma enhanced chemical vapor disposition. This process includes the steps of introducing the metacarborane into a chamber that contains the substrate, energizing the metacarborane, dehydrogenation of the metacarborane (i.e., deprotenation or causing hydrogen loss) to form a natural n-type boron carbide semiconductor polytype. Where the metacarborane is introduced at a pressure of about 60 mTorr, the process may further include introducing an inert gas source, such as, but not limited to, argon, krypton, neon and xenon, into the chamber to supply additional background pressure of about 240-640 mTorr. Further, nickelocene (Ni(C5H5)2), chromocene (Cr(C2H5)2), ferrocene (Fe(C5H5)2), cobaltocene (Co(C2H5)2) or manganocene (Mn(C5H5)2)) may also be introduced into the chamber to make the boron carbide semiconductor of the present invention even more n-type.